The bandwidth shortage increasingly experienced by mobile carriers has motivated the exploration of the underutilized millimeter wave (mmW) frequency spectrum between 6G and 300G Hz for the next generation broadband cellular communication networks. The available spectrum of mmW band is two hundred times greater than the conventional cellular system. The mmW wireless network uses directional communications with narrow beams and can support multi-gigabit data rate. The underutilized bandwidth of the mmW spectrum has wavelengths ranging from 1 mm to 100 mm. The very small wavelengths of the mmW spectrum enable large number of miniaturized antennas to be placed in a small area. Such miniaturized antenna system can produce high beamforming gains through electrically steerable arrays generating directional transmissions.
With recent advances in mmW semiconductor circuitry, mmW wireless system has become a promising solution for the real implementation. The main characteristics of MMW are short wavelength/high frequency, large bandwidth, high interaction with atmospheric constituents and high attenuation through most solid materials. This leads to a sparse-scattering environment and noise-limited system. Beamforming is the key to compensate channel attenuation and reduce interference in MMW networks. However, the heavy reliance on directional transmissions and the vulnerability of the propagation environment present particular challenges for the MMW network. For example, the MMW channel changes much faster than today's cellular system due to the small coherence time, which is about hundreds of microsecond. The MMW communication depends extensively on adaptive beamforming at a scale that far exceeds current cellular system. Further, the high reliance on the directional transmission introduces new issues for synchronization. Broadcast signals may delay the base station detection during cell searching for initial connection setup and for handover because both the base station and the mobile station need to scan over a range of angles before the mobile station can detect the base station. Furthermore, the MMW signals are extremely susceptible to shadowing. The appearance of obstacles, such as human bodies and outdoor materials would cause the signal outage. The small coverage of the MMW cell causes the relative path loss and the cell association to change rapidly. Resolving frequent intermittent connectivity loss and enabling rapid adaptable communication is one of the key features to the development of the MMW wireless network.
Since the MMW bands cannot penetrate obstacles very well and very sensitive to non-light of sight (NLOS) communication and other impairments such as absorption by foliage, rain and other particles in air, the microwave band can improve coverage and ensure seamless user experience in mobile applications. The heterogeneous deployment of both macro cells and MMW small cells can be considered. The macro cell works on the microwave bands used by legacy communication technologies such as E-UTRAN. The deployment is used to overcome the physical limitations of MMW through exploiting both microwave and MMW bands simultaneously and facilitating coexistence of several communication layers with different coverage. Important control messages and signals can be transmitted via the microwave bands to guarantee the transmission reliability.
Improvements and enhancements are required multiple connectivity for the heterogeneous network.